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Duckweed

These images illustrate the above-surface and under-surface anatomy of duckweed (Lemna spp.) as well as the typical environment in which these fast-growing plants -- the smallest flowering plants in the world -- are found.

Duckweed, the tiny aquatic plant that carpets the surface of many ponds and lakes in summer, has gained importance in recent years. Biologists have long understood the role of duckweed in natural ecosystems. Now scientists are beginning to harness the ability of duckweed to expand its population size at near unfathomable rates given the right conditions.

Like most aquatic plants, duckweed thrives when nitrogen and phosphorus concentrations are high. The tiny plants absorb these nutrients from the water through their roots, just as terrestrial plants absorb fertilizer from the soil. They grow, photosynthesize, and reproduce. They also compete with algae for precious nutrients and sunlight. This is important because if nitrogen and phosphorus levels are very high and duckweed is absent in an ecosystem, algae can become overabundant. Oxygen that comes from the algae is lost when the plants die and decay. If large enough, algal blooms can lower oxygen levels to the point where fish and other aquatic life die.

In recent decades, nitrogen and phosphorus levels in ponds and lakes around the world have risen dramatically. Human activities are largely responsible for this increase. Untreated sewage, agricultural runoff, and industrial waste frequently pour into ponds, lakes, and streams, and duckweed is not always there to absorb the volume of nitrogen and phosphorus these pollutants contain. Increasingly, though, scientists are introducing the tiny plants into aquatic ecosystems and using them as veritable sponges to clean up contaminated environmentsor, better still, to reduce nitrogen and phosphorus levels in wastewater before it is released into the environment.

To be of any long-term benefit to the environment, the nutrient-rich duckweed plants must be skimmed off a pond or lake before they die; otherwise they will simply release the nitrogen and phosphorus they contain back into the water as they decay. It's a big job, but some people and are finding the task worthwhile, given that duckweed is a relatively cheap and, not surprisingly, nutrient-rich feed for livestock and fish.